Effects of surface treatments and annealing on carbon-based molecular sieve membranes for gas separation

A new method in preparing carbon-based molecular sieve (CMS) membranes for gas separation has been proposed. Carbonbased films are deposited on porous Al2O3 disks using hexamethyldisiloxane (HMDSO) by remote inductively coupled plasma (ICP) chemical vapor deposition (CVD). After treating the film with ion bombardment and subsequent pyrolysis at a high temperature, carbon-based molecule sieve membranes can be obtained, exhibiting a very high H2/N2 selectivity around 100 and an extremely high permeance of H2 around 1.5 10 6 mol m 2 s 1 Pa 1 at 298 K. The O2/N2 selectivity could reach 5.4 with the O2 permeance of 2 10 7 mol m 2 s 1 Pa 1 at 423 K. During surface treatments, HMDSO ions were found to be more effective than CH4, Ar, O2 and N2 ions to improve the selectivity and permeance. Short and optimized surface treatment periods were required for high efficiency. Without pyrolysis, surface treatments alone greatly reduced the H2 and N2 permeances and had no effect on the selectivity. Besides, without any surface treatment, pyrolysis alone greatly increased the H2 and N2 permeances, but had no improvement on the selectivity, owing to the creation of large pores by desorption of carbon. A combination of surface treatment and pyrolysis is necessary for simultaneously enhancing the permeance and the selectivity of CMS membranes, very different from the conventional poreplugging mechanism in typical CVD.